(a) Technical Field
The present disclosure relates to a method of controlling motor torque of a hybrid electric vehicle. More particularly, it relates to a method of controlling torque of a driving motor in a coasting state of a hybrid electric vehicle.
(b) Background Art
In general, a hybrid electric vehicle, which is driven by an engine and a motor, is an environment-friendly vehicle discharging less exhaust gas with improved fuel efficiency through the use of energy from fossil fuel in combination with electric energy. In this regard, FIG. 1 is a diagram schematically showing the configuration of a powertrain including an engine and a motor, which are power sources for driving, and a transmission, in a hybrid electric vehicle. As shown in FIG. 1, a powertrain including a drive train, such as a power transmission device, in a hybrid electric vehicle includes: an engine 1 and a motor 3 (hereafter referred to as a driving motor) which are arranged in series as driving sources for driving the vehicle; an engine clutch 2 selectively connecting or disconnecting power between the engine 1 and the driving motor 3; an inverter 5 for driving and controlling the driving motor 3; a transmission 4 changing and transmitting power from the engine 1 and the driving motor 3 to a driveshaft; and a hybrid starter generator (HSG) 7 connected to the engine 1 in order to transmit the power and start the engine or generate electricity from the power from the engine. A chargeable/dischargeable battery 6 that is the power source (i.e., electric power source) for the driving motor 3 is connected to the driving motor 3 through the inverter 5.
In this configuration, the engine clutch 2 selectively connects or disconnects power between the engine 1 and the driving motor 3 by being engaged or disengaged by hydraulic pressure. The transmission 4 is connected to the output side of the driving motor 3 and transmits the power from the engine and the driving motor to the driveshaft. The inverter 5 converts the direct current of the battery 6 into alternating current and applies the alternating current to the driving motor 3 to drive the driving motor 3.
Driving modes of common hybrid electric vehicles are selected in accordance with the driving conditions. For instance, hybrid electric vehicles can be driven in an Electric Vehicle (EV) mode that is a pure electric vehicle mode using only the power from driving motor 3 and an Hybrid Electric Vehicle (HEV) mode that uses power from both of the engine 1 and the driving motor 3. Further, when the vehicles are braking (i.e., a brake pedal is depressed) or coasting by inertia, a regenerative braking (RB) mode for charging the battery 6 with the braking and inertia energy that is recovered by electric generation of the driving motor 3 can be performed. The hybrid starter generator 7 also charges the battery 6 by operating as a power generator using the power from the engine 1 or operating as a power generator in a regenerative braking condition. For reference, in hybrid electric vehicles, when the driving motor 3 is connected to the transmission 4, it is called a Transmission Mounted Electric Device (TMED) type.
FIG. 2 is a diagram showing characteristics of creep torque and coasting torque in a hybrid electric vehicle and schematically showing an example of the state of torque (e.g., creep torque, coasting torque, etc.) of a driving motor according to a motor speed (i.e., speed of the input shaft of a transmission). As known in the art, hybrid electric vehicles creep or coast with a brake pedal-off and an accelerator pedal-off state. The term ‘pedal-off’ means the state when a driver does not operate the corresponding pedal, i.e., a driver takes a foot off the pedal, while the term ‘pedal-on’ means the state when a driver depresses the corresponding pedal.
The creep torque is implemented by an idle control characteristic of an engine and a torque converter of an automatic transmission, when a driver takes feet off a brake pedal and an accelerator pedal, i.e., in brake pedal-off and accelerator pedal-off state. In a creep period requiring creep torque, a driving force is generated by engine torque for maintaining an idling speed at a low speed.
In a coasting period at a relatively high speed, a braking force is generated by a friction force of the engine in a fuel-cut state, so the vehicle is braked by engine friction torque (i.e., engine braking torque). The torque of the driving motor in coasting (hereafter referred to as ‘coasting torque’), which is negative torque, is charging torque (e.g., power generation torque, regenerative braking torque, etc.) that generates a braking force and makes the battery be charged. In a TMED hybrid electric vehicle with a driving motor connected to an automatic transmission, there is no torque converter and idle control characteristic is different, but there is a need for implementing driving comfort the same as that of common vehicles equipped with an automatic transmission.
FIGS. 3 and 4 are diagrams illustrating coasting torque control of a TMED hybrid electric vehicle. FIG. 3 shows a state when coasting torque is obtained by a regenerative force of the driving motor 3 in an EV mode in which a vehicle is driven by only the driving motor 3 with the engine clutch 2 disengaged. FIG. 4 shows a state when coasting torque is obtained by friction torque of an engine 1 with the engine clutch 2 engaged.
Referring to FIG. 3, when a vehicle running in an EV mode with an engine clutch disengaged starts to coast using the inertia of the vehicle without a brake pedal and an accelerator pedal depressed, regenerative power (e.g., charging power) by the driving motor 3 is stored in the battery 6 through the inverter 5. When a TMED hybrid electric vehicle is in a normal situation, as described above, the driving torque is in charge of coasting torque in the EV mode to increase driving efficiency and converts the inertia energy of the vehicle into electric energy. The motor torque (i.e., coasting torque) is controlled at engine friction torque corresponding to the current speed of the input shaft of the transmission, whereby the input speed of the transmission determines the motor speed.
When the battery 6 is fully charged, the engine clutch 2 is engaged and a braking force is generated from a friction force of the engine 1 in a fuel-cut state, as shown in FIG. 4. However, the inertia force of the vehicle depends on the rotational inertia of the engine before/after the engine clutch 2 is engaged, so there is a difference in deceleration before and after the engine cutch is engaged, when the vehicle coasts.
In more detail, first, the deceleration {dot over (ω)}EV of a vehicle before an engine clutch is engaged (i.e., when an engine clutch is not engaged) in coasting can be obtained from the following equation (1).(JMot+JDT+JVeh)×{dot over (ω)}EV=τMot+τDT_Drag+τLoad    (1)
When the torque τMot of a driving motor is controlled at engine friction torque τEng_Drag, that is, τMot=τEng_Drag, as in the related art, the deceleration {dot over (ω)}EV can be obtained from the following equation (2).
                                          ω            .                    EV                =                                            τ              Eng_Drag                        +                          τ              DT_Drag                        +                          τ              Load                                                          J              Mot                        +                          J              DT                        +                          J              Veh                                                          (        2        )            
In contrast, the deceleration {dot over (ω)}HEV of a vehicle with an engine clutch engaged can be obtained from the following equation (3).(JEng+JMot+JDT+JVeh)×{dot over (ω)}HEV=τEng_Drag+τDT_Drag+τLoad   (3)
The deceleration {dot over (ω)}HEV can be obtained from the following equation (4), because the rotational inertia JEng of an engine is added.
                                          ω            .                    HEV                =                                            τ              Eng_Drag                        +                          τ              DT_Drag                        +                          τ              Load                                                          J              Eng                        +                          J              Mot                        +                          J              DT                        +                          J              Veh                                                          (        4        )            
In the equations (1) and (2), JEng is the rotational inertia of an engine, JMot is the rotational inertia of a motor, JDT is the rotational inertia of a drive train, and JVeh is the rotational inertia of a vehicle. Further, τMot is motor torque, τEng_Drag is engine friction torque, τDT_Drag is friction torque of a drive train, and τLoad is driving load torque of a vehicle.
Further, {dot over (ω)}EV is the deceleration of a vehicle coasting without an engine clutch engaged and {dot over (ω)}HEV is the deceleration of a vehicle coasting with an engine clutch engaged. All of the rotational inertia, torque, and deceleration are values converted on the basis of the input shaft of the transmission.
As a result, as shown in equations (2) and (4), a difference in deceleration of a vehicle that is coasting, before and after an engine clutch is generated, as in the following equation (5).{dot over (ω)}EV≠{dot over (ω)}HEV   (5)
The difference in deceleration causes rapid speed change of a vehicle, and a driver feels a difference when he/she shifts, due to the difference in inertia.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure, and therefore, it may contain information that does not form the related art that is already known in this country to a person of ordinary skill in the art.